As used in this specification, the term “composite material” means a combination of a binding matrix material and reinforcing fibre material extending through the matrix. The term includes any fibre-reinforced plastic material. The matrix material can comprise any suitable binding material, including thermosetting resins such as epoxy or polyester resins, or certain thermoplastic materials for example, and the reinforcing fibre material can comprise one or more of, for example, glass fibre, carbon fibre, polyethylene, poly(p-phenylene-2,6-benzobisoxazole) (PBO) and other synthetic and organic fibres.
Composite tubes may be created by a variety of manufacturing methods, including a technique known as filament winding. In filament winding, the fibre and resin material are wound in a helical fashion about a mandrel and the composite tube results from the coverage of the mandrel by the composite material. The end-fittings may be metal or any other material suitable for the application.
Compared to tubes made of metal such as steel, composite tubes are considerably lighter, have equivalent or higher strength, and generally have better fatigue resistance. Furthermore, the composite tubes do not corrode which is advantageous in caustic environments or undersea applications for example. Another advantage of composite tubes over metal tubes in some applications is that metal tubes burst by fracturing, whereas composite tubes burst by delaminating, in a slow rupturing process.
A disadvantage of composite tubes over conventional metal tubes, however, is that the introduction of load into the ends of composite tubes can be problematic, particularly when the load is high. Ideally the connection to the ends of the composite tube would be at least as strong as the body of the tube.
The problem of connecting end-fittings to the composite tube may be avoided by using external metal tie-rods to secure the end-fittings; hence the composite tube itself does not carry the axial load emanating from the end-fitting. However the use of external tie-rods results in a relatively bulky and heavy assembly, and the tie-rods can stretch when the end-fittings are subjected to high pressures.
One method for securing end-fittings to the composite tube is with adhesives. Adhesive bonding is however relatively weak as it relies on the shear strength of the adhesive to transfer the load between the composite tube and the end-fitting. Adhesively bonded end-fittings in applications of composite hydraulic cylinders, for example, have proven unsatisfactory due to excessive leakage or bursting in high pressure applications.
Another method for securing end-fittings to composite tubes is to engage differing diameters on the two parts. A conventional mechanical thread is one realisation of this method; however threaded connections in the composite material are not suitable for applications involving high loads, because cutting the fibres as well as the matrix considerably reduces the composite material's structural strength.
Alternatively, an annular barb on the end-fitting can be provided to engage with an annular groove on the composite tube. The strength of the connection between end-fitting and composite tube is limited by the shear strength of the barb and the strength of the groove in the composite tube.
In another method, the composite tube may be flared, creating a tapered interface at which the composite tube and end-fitting engage. The axial force at the tapered interface resolves into a force normal to the taper and a shear force parallel to the taper. Additionally friction is generated at the interface, further increasing the shear force. Such designs typically show weakness at the point where the shear force is first introduced into the composite tube and where the fibres are compressed perpendicular to the fibre direction by the force normal to the taper.
In yet another method the composite tube is created by laying fibres axially along the length of tube and over a shoulder on the end-fitting. The axial fibres terminate at the ends of the tube. To prevent the ends of the axial fibres from pulling back over the shoulder when load is applied to the end-fitting, additional fibres are wound circumferentially over the axial fibres in the region of the shoulder. The axial fibres will pull out from between the circumferential fibres and the shoulder if the applied load is high enough, hence the strength of the connection between end-fitting and composite material is limited to the shear strength between the laminates.
In a further method, fibres are again laid axially along the length of the tube, but rather than terminating at the end of the tube, the fibres transition from an axial orientation to a circumferential orientation as they are wound over the shoulder. After a suitable angle of rotation of the fibre about the neck adjacent to the shoulder, the fibre is wound back over the shoulder and along the length of the tube. The process of winding over the shoulder is repeated at the opposite end of the tube. The connection is relatively strong, however winding the fibres over the shoulder can be problematic due to the fibre slipping on the shoulder as it transitions from an axial to circumferential orientation. Slippage can be minimised by following a geodesic path, however defining the geodesic path over an arbitrary geometry shoulder and then laying fibres along this path is difficult, and the resulting fibre orientation does not provide optimal strength. Another difficulty encountered is fibre build-up occurring at the neck.
In a similar method the transition from an axial orientation with respect to the tube, to a circumferential orientation is effected by winding the fibre about pins or tabs on the end-fitting. After a suitable angle of rotation of the fibre about the tube in the circumferential direction, the fibre transitions back to an axial orientation, again using the pin or tab to effect the direction change. The use of the pins or tabs reduces the problem of the fibre slipping at the turn-around region and avoids the need to follow a geodesic winding path. However this variation suffers from fibre build-up in the turn-around region and the strength of the connection is compromised by the abrupt change in fibre direction at the pin or tab.